Best way to keep interior smelling fresh

There are some closet size dehumidifiers which might be helpful. Do a search and read the reviews.

This one does not require power or batteries, you don't have to empty it but you "recharge" it. Some of my tear drop friends use one similar to this when they camp to control condensation.

www.amazon.com/dp/B00ANX1PAI?psc=1

I put a solar vent in my closet. It moves just enough air to keep it circulating. I'm not suggesting this because I can't tell you for sure if it actually will help. Put it's pretty cool

The 40 watt lamp is the best idea here so far. A dehumidifier is good in a very humid area but requires constant maintenance and if it over flows you are screwed. One or two of the light bulbs will keep the humidity down. Removing as much of the bedding, curtains and other types of cloth materials that hold smells and moisture helps a lot too. Store all that stuff in plastic bags. The kind you suck the air out of and only bring them out when you plan to use the trailer. It's a pain in the butt of course but well worth the effort. Keeping windows or vents cracked open is a bad idea. Your paneling will act like a sponge and suck the moisture in when it's humid and then when it dries up outside, so will the paneling. All that action going back and forth will eventually de-laminate the paneling. So I would get it dry and then shut everything up, turn on the light bulb and leave it at that till the next time you open it. Open it up on a nice day when it's not too humid and let it breath. Then shut it up at the end of the day and turn the light bulb back on.

Thanks all for your recommendations. I am already using one of those Damp Rid containers that you fill up with the crystals and that helps quite a bit. Can still get the one that hangs as Turbodaddy mentioned. Sold to be hung in a closet or something like that. You're right , they do smell but must have downgraded them some as not as toxic. Going to do a combo of what everyone suggested and close her up, turn on a lamp with a 40 watt bulb, keep my damp rid bucket filled up and keep an eye out. BTW, Palmetto bugs are the state bird of South Carolina ( just kidding) but big cockroaches with wings are a real treat !!!
Thanks again,
Mike

Relative humidity is the main issue here. Mold and mildew spores are airborne, there is no way to completely avoid them. Bleaches and other surface treatments that kill existing spores will eventually break down and are only a short term solution. The best you can do is to try controlling the environment inside the trailer. Mold and mildew need a consistently wet environment to grow. Ideally, you need a constant 60% or less humidity level to keep water from condensing. Practically, this is impossible to maintain in our old trailers, especially so on the east coast with your 90% plus outdoor humidity levels.

If the humidity level inside the trailer is higher than that of the outside air, water will condensate on surfaces, creating an environment for mold and mildew. Warm air is like a sponge, the warmer the air, the more water it can hold. If the air temperature inside the trailer is warmer than the outside air, it will contain more moisture relative to the outside air, creating a higher humidity level inside the trailer.

Closing up a trailer makes sense in the desert where the dry air is sucking the moisture out of anything it can. However, in your situation, with your high outdoor humidity levels, closing up the trailer will actually make your problem worse as the warmer air inside the trailer will draw moisture from the outside air. With the windows and vents buttoned up, the moist air is trapped inside the trailer and condensates, creating the very environment you want to avoid.

Without a dehumidifier, about the best you can do is try to equalize the inside and outside air temperature and humidity levels, hence cracking open the windows and roof vent. The light bulb does not reduce the humidity in the trailer. Just the opposite. It slightly raises the air temperature inside the trailer so it holds more water before condensing. This is more so at night when it is cooler outside. Warm air also raises, this is convection. An open roof vent allows the warmer, moister air to escape, but only if a lower window is cracked to allow cooler outside air in to replace it. The light bulb does not dry out the air, that's impossible, it only facilitates the air convection and is most effective if placed on the floor.

Moving air is less apt to condensate than stationary air, so a low speed recirculating fan will help. A desiccant is just insurance. Remove your fabrics, cushions, window coverings, etc. Open up all the cabinet doors. All of this will help keep your trailer at a relatively stable moisture level so your wood will not be subjected to the wet dry cycles that cause problems.

Keeping the trailer in the shade, under your house as you do is the best possible place for it as the temperature swings are less. When all is said and done, like chamtzoo said, you are in a tough situation. You will likely always have to do a wipe down from time to time. At least it will be easier with everything removed from the trailer.

Dave, I don't recall if your trailer has been restored or not, but if you have any rotten wood, the musty smell will never go away.